Yepoleb: Can you elaborate what exclusive features you'd like to see work offline? 
 Gekko_Dekko: offline achievements would be nice, if possible (I dunno how to describe it better, lol. Overall - the same list of unlockables, but accessible and fully workable offline).  
 Also ability to disable any unwanted online connectivity (so - people, who dont use social features, will be able to use this client as alternative for deprecated gogdownloader). Not altogether, but configurable, obviously. 
 I know that it wont work for games themself, 
due to way, gog's libraries currently work. But for client - shouldnt be difficult.  
 Looking for future, you may also implement something like wine support and such, but thats will be a bit hard. Good luck, overall :) 
 For features that are easily disabled like the presence service I'll probably put in a setting, but most offline stuff would require extra code and won't be a priority for now unfortunately. Focus is currently multiplayer, installing native games and social stuff in that order. I know social features are a bit controversial, but in my opinion they're necessary to make the client attractive to new users.